Remote wireless unit having reduced power operating mode for a discrete multitone spread spectrum communications system

ABSTRACT

A remote unit for a personal wireless area network includes a receiver, an AC power supply, a battery-backup power supply and a controller. The battery-backup becomes operative when the AC power supply fails and supplied power to the receiver. The controller detects when the AC power supply fails and controls the receiver and the battery-backup power supply by invoking a sleep mode of operation. The sleep mode of operation is periodically interrupted by the controller controlling the receiver and the battery-backup power supply to enter a standby mode of operation in which the receiver scans for a CONNECT message from a base station indicating an incoming call. The controller coordinates the sleep mode and the standby mode of operations based on a frame count that is generated from an identification number of the remote unit. A highly bandwidth-efficient communications method is employed in the base station to enable it to coordinate communication with the remote unit when it changes from the sleep mode to the standby mode.

This patent application is a Continuation-In-Part of the U.S. patentapplication by David Gibbons, et al. entitled “REMOTE WIRELESS UNITHAVING REDUCED POWER OPERATING MODE”, Ser. No. 08/796,586, filed Feb. 6,1997, now U.S. Pat. No. 6,085,114 and assigned to AT&T Wireless ServicesInc.

CROSS-REFERENCES TO RELATED APPLICATIONS

The invention disclosed herein is related to the co-pending U.S. patentapplication by Siavash Alamouti, Doug Stolarz, and Joel Becker, entitled“VERTICAL ADAPTIVE ANTENNA ARRAY FOR A DISCRETE MULTITONE SPREADSPECTRUM COMMUNICATIONS SYSTEM”, Ser. No. 08/806,510, filed on the sameday as the instant patent application, assigned to AT&T WirelessServices Inc., and incorporated herein by reference.

The invention disclosed herein is related to the copending U.S. patentapplication by Alamouti, et al., entitled “Method for Frequency DivisionDuplex Communications”, Ser. No. 08/796,584, filed Feb. 6, 1997,assigned to AT&T Wireless Services, and incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to improvements to communications systems.More particularly, the present invention relates to wireless discretemultitone spread spectrum communications systems.

2. Description of the Related Art

Wireless communications systems, such as cellular and personalcommunications systems, operate over limited spectral bandwidths andmust make highly efficient use of the scarce bandwidth resource forproviding good service to a large population of users. A Code DivisionMultiple Access (CDMA) protocol has been used by wireless communicationssystems for efficiently making use of limited bandwidths and uses aunique code for distinguishing each user's data signal from data signalsof other users. Knowledge of the unique code with which any specificinformation is transmitted permits separation and reconstruction of eachuser's message at the receiving end of the communication channel.

Adaptive beamforming technology has become a promising technology forwireless service providers for offering large coverage, high capacity,and high quality service. Based on this technology, a wirelesscommunication system can improve its coverage capability, systemcapacity, and performance significantly. A personal wireless accessnetwork (PWAN) system, described in the cross-referenced Alamouti,Stolarz, et al. patent applications, uses adaptive beamforming combinedwith a form of the CDMA protocol known as discrete multitone spreadspectrum (DMT-SS) for providing efficient communications between a basestation and a plurality of remote units (RUs).

The remote units are powered primarily from AC power sources and includea battery for providing battery backup power when AC power fails. Toconserve battery power, an RU has a sleep mode of operation withperiodic power-up modes for checking whether any calls are attempting tobe connected to the RU. When an RU is in a sleep mode, it expedient thatthe system operate in such a way so that appropriate actions are takenfor completing a call to a sleep mode RU.

One approach for ensuring that calls are completed to a remote unitoperating in a sleep mode is to maintain a database at a centrallocation that stores the current operating mode of each remote in thesystem. When a remote unit enters a sleep mode of operation, the remoteunit reports the change of operational status to the database.Similarly, the remote unit reports a change of status back to a standbyoperating mode. This approach has a drawback when a number of remoteunits recorded in the database experience frequent power outages. Insuch a situation, recording, managing and synchronizing power outageinformation in the database is particularly cumbersome when the databaseis large, perhaps holding status information for 3 to 4 thousand remoteunits. This drawback is further compounded when the database isduplicated multiple times throughout the system. When several thousandsubscribers experience a power outage and AC power is restored beforethe database has completed recording the power outage, a databaseapproach becomes unwieldy. Another complicated situation is whenmultiple remote units lose power at the same time. The affected remoteunits cannot all access the channel simultaneously for communicatingtheir status to the database. A collision avoidance scheme must beimplemented that spans a period of time and that is open for thepossibility of power being restored before the database has beencompletely revised.

This approach has another drawback in that a remote unit entering thesleep mode consumes system bandwidth in notifying the database. FIG. 4shows an exemplary flow of internal messaging that occurs betweenvarious layers of a remote unit when loss of AC power is detected and adatabase is notified of the operational status change. Time is shownalong the vertical axes of FIG. 4, with advancing time being indicatedtoward the bottom of FIG. 4. In FIG. 4, four layers of the remote unitoperating system are shown: Health; OAM&P (Operations, Administration,Maintenance & Provisioning), MAC (Media Access Control) and physical.Only MAC layer of the base station is shown. At 40, AC power failure isdetected by the Health layer. At 41, an EVENT message is sent from theHealth layer to the OAM&P layer indicating that AC power has failed. TheOAM&P layer sends an ACTION message to the MAC layer at 42. The MAClayer responds at 43 by sending an ACTION_RSP message to the OAM&P layerindicating that base station notification is pending. At 44, the MAClayer waits a random length period of time before sending an unsolicitedCAC message at 45 to the MAC layer of the base station indicating theneed for the remote unit to enter the sleep mode. At 46, the MAC layerof the base station sends an acknowledgment message to the MAC layer ofthe remote unit acknowledging receipt of the unsolicited CAC message. Inresponse, the MAC layer of the remote unit sends an EVENT message at 47to the OAM&P layer that the notification is done. The OAM&P layer firstsends an EVENT message to the MAC layer indicating that the sleep modehas been entered at 48, and then sends a message at 49 to the physicallayer to power down.

What is needed is a way for a PWAN system to be aware that a remote unitis operating in a sleep mode so that appropriate actions can be taken bythe system so that calls can be completed to a remote unit operating ina sleep mode.

SUMMARY OF THE INVENTION

The present invention provides a method for reducing power consumptionof a remote unit in a PWAN system. A remote unit is powered using abattery backup power supply when an AC power supply fails at the remoteunit. A sleep mode of operation is entered at the remote unit that has areduced power consumption for the battery backup power supply. Theremote unit is synchronized to a TDD timing structure a predeterminedperiod of time after entering the sleep mode of operation. A standbymode of operation is then entered at the remote unit in which a CONNECTmessage indicating an incoming call for the remote unit is scanned forby the receiver. When no CONNECT message is received, the remote unitreenters the sleep mode of operation. According to the invention, thepredetermined period of time is a predetermined number of subframesafter a boundary subframe of the TDD timing structure. Preferably, thepredetermined number of subframes is based on an identification numberof the remote unit.

The present invention also provides a remote unit for a personalwireless area network that includes a receiver, an AC power supply, abattery-backup power supply and a controller. The battery-backup becomesoperative when the AC power supply fails and supplies power to thereceiver. The controller detects when the AC power supply fails andcontrols the receiver and the battery-backup power supply by invoking asleep mode of operation. The sleep mode of operation is periodicallyinterrupted by the controller controlling the receiver and thebattery-backup power supply to enter a standby mode of operation inwhich the receiver scans a CONNECT message indicating an incoming call.The controller coordinates the sleep mode and the standby mode ofoperations based on a frame count that is generated from anidentification number of the remote unit.

In accordance with another aspect of the invention, a highlybandwidth-efficient communications method is disclosed for the basestation to enable it to communicate with a remote unit that is in thesleep mode. The remote unit has a unique identification value that isdifferent from the identification value of other remote units that maybe communicating with the base station. The base station begins byestablishing a periodic reference instant at the base station and at theremote station. Then the base station determines a delay intervalfollowing the periodic reference instant at the base station, the delayinterval being derived from the unique identification value of theremote unit. The base station receives spread signals from the remoteunits with which it communicates, each comprising an incoming datatraffic signal spread over a plurality of discrete traffic frequencies.The base station adaptively despreads the signals received it receivesby using despreading weights. The base station attempts to initiate acommunication with the remote unit that is currently in the sleep mode.If the attempting step fails to initiate communications with the remoteunit, the base station concludes that the remote unit is in the sleepmode. In response to this, the base station waits for the delay intervalfollowing the periodic reference instant at the base station beforetransmitting to the remote unit. The base station then transmits to theremote unit a spread signal comprising an outgoing data traffic signalspread over a plurality of discrete traffic frequencies. The remote unithas simultaneously changed from the sleep mode to the standby mode andis able to receive and respond to the spread signal transmitted from thebase station.

In accordance additional aspects of the invention, the base station ispart of a wireless discrete multitone spread spectrum communicationssystem. Further, the periodic reference instant is established by abeginning subframe count instant that is incremented by a packet countvalue at the base station and at the remote unit. In addition, the delayinterval is determined by a value N of a quantity of M least significantbits of the unique identification value of the remote unit, the delayinterval being an interval required for the occurrence of a plurality ofN of the beginning subframe count instants. The resulting inventionenables the base station to be aware that a remote unit is operating ina sleep mode so that appropriate actions can be taken by the basestation to assure that calls can be completed to the remote unit.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the accompanying figures in which like reference numeralsindicate similar elements and in which:

FIG. 1 is an architectural diagram of the PWAN system, including remotestations transmitting to a base station;

FIG. 2 is an architectural diagram of the remote station X as a sender;

FIG. 3 is an architectural diagram of the remote station X as areceiver;

FIG. 4 shows an exemplary messaging flow occurring between variouslayers of an exemplary remote unit and through an airlink to a basestation when a loss of AC power at the remote unit is detected;

FIG. 5 shows a message flow sequence for a terminating call for thesituation when a target remote unit is operating in the sleep mode;

FIG. 6 shows a sequence of events with respect 6 ms subframe structureof the present invention;

FIG. 7 is an exemplary graph showing Battery Operating Time, measured inhours, for Sleep Mode Duty Cycle (:1); and

FIG. 8 is an architectural diagram of the base station Z.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an architectural diagram of the personal wireless accessnetwork (PWAN) system described in the referenced Alamouti, Stolarz, etal. patent applications and which is the environment of the presentinvention. Two users, Alice and Bob, are located at a remote stationunit, or remote unit (RU), X and wish to transmit their respective datamessages to a base station Z. Remote unit X is positioned to beequidistant from each of antenna elements A, B, C, and D at base stationZ. Two other users, Chuck and Dave, are located at a remote station unitY and also wish to transmit their respective data messages to basestation Z. Remote unit Y is geographically different from remote unit Xand is not equidistant from each of antenna elements A, B, C, and D ofbase station Z. Remote units X and Y, and base station Z use a form ofthe CDMA protocol known as discrete multitone spread spectrum (DMT-SS)which is used for providing efficient communications between basestations and remote units. The DMT-SS protocol is indicated in FIG. 1 asa multi-tone CDMA.

In the DMT-SS protocol, a user data signal is modulated by a set ofweighted discrete frequencies or tones. The weights are spreadingweights that distribute the data signal over many discrete tonescovering a broad range of frequencies. The weights are complex numbershaving a real component that is used for modulating the amplitude of atone and a complex component that is used for modulating the phase ofthe same tone. Each tone in the weighted-tone set bears the same datasignal. Plural users at a transmitting station can use the same tone setfor transmitting their data, but each of the users sharing the tone sethas a different set of spreading weights. The weighted-tone set for aparticular user is transmitted to the receiving station where it isprocessed with despreading weights that are related to the user'sspreading weights for recovering the user's data signal. For each of aplurality of spatially separated antennas at the receiver, the receivedmultitone signals are transformed from time-domain signals tofrequency-domain signals. Despreading weights are assigned to eachfrequency component of the signals that are received by each antennaelement. The values of the despreading weights are combined with thereceived signals for obtaining an optimized approximation of individualtransmitted signals characterized by a particular multitone set andtransmitting location.

The PWAN system has a total of 2560 discrete tones (carriers) that areequally spaced in 8 MHZ of available bandwidth in the frequency range of1850 to 1990 MHZ, with a spacing between the tones of 3.125 KHz. Thetones are used for carrying traffic messages and overhead messagesbetween the base station and the plurality of remote units. The totalset of tones are numbered consecutively from 0 to 2559, starting fromthe lowest frequency tone. The tones used for traffic messages aredivided into 32 traffic partitions, with each traffic channel requiringat least one traffic partition of 72 tones.

The overhead message tones are used for establishing synchronization andfor passing control information between base stations and remote units.A Common Link Channel (CLC) is used by a base station for transmittingcontrol information to remote units. A Common Access Channel (CAC) isused by a remote unit for transmitting messages to the base station.There is one grouping of tones assigned to each channel. The overheadchannels are used in common by all remote units when control messagesare exchanged with a base station.

Transmission from a base station to a remote unit is called “forwardtransmission” and transmission from a remote unit to a base station iscalled “reverse transmission”. Time Division Duplexing (TDD) is used bybase stations and remote units for transmitting data and controlinformation in both directions over the same multi-tone frequencychannel. The time between recurrent transmissions in either direction iscalled a TDD period which, is equal to 3 ms. For every TDD period, thereare four consecutive transmission bursts in each direction. Data istransmitted during each burst using multiple tones. The base station andeach remote unit synchronize and conform to a TDD timing structure andframing structure that has 1 frame equal to 8 subframes and 1 subframeequal to 2 TDD periods. A superframe is 256 subframes, or 1536 ms. Allremote units and base stations are synchronized such that all remoteunits transmit simultaneously and then all base stations transmitsimultaneously. When a remote unit initially powers up, it acquiressynchronization from a base station so that control and traffic messagescan be exchanged within the prescribed TDD time format. A remote unitmust also acquire frequency and phase synchronization for the DMT-SSsignals so that the remote unit is operating at the same frequency andphase as an associated base station.

Selected tones within each tone set are designated as pilot tones thatare distributed throughout the frequency band and carry known datapatterns for enabling an accurate channel estimation. A series of pilottones, having known amplitudes and phases, are spaced apart in frequencyby approximately 30 KHz for providing an accurate representation of achannel response over the entire transmission band, that is, theamplitude and phase distortion introduced by the communication channelcharacteristics over the transmission band.

FIG. 2 shows an architectural diagram of remote station X operating as asender station. Alice and Bob each input data to remote station X. Thedata is sent to a vector formation buffer 202 and also to a cyclicredundancy code generator 204. Data vectors are output from buffer 202to a trellis encoder 206. The data vectors are in the form of a datamessage formed by concatenating a 64 K-bit data block with a seriallyassigned block number. CRC generator 204 generates LCC vectors that areoutput to trellis encoder 206. The LCC vectors are in the form of anerror detection message formed by concatenating a CRC value with theserially assigned block number of the data block. The trellis encodeddata vectors and LCC vectors are then output to a spectral spreadingprocessor 208. The resultant data tones and LCC tones are then outputfrom processor 208 to a transmitter 210 for transmission to the basestation.

The personal wireless access network (PWAN) system described in thecross-referenced Alamouti, Stolarz, et al. patent application provides amore detailed description of a high-capacity mode, where one trafficpartition is used in one traffic channel. A base station transmitsinformation to multiple remote units that are located in the basestation's cell. The transmission formats are for a 64 Kbps trafficchannel, together with a 4 Kbps Link Control Channel (LCC) between thebase station and a remote unit. A binary source, for example, Alice orBob, delivers data, or information bits, to a sender transmitter at 64Kbits/sec. This translates to 48 bits in one transmission burst. Theinformation bits are encrypted according to a triple data encryptionstandard (DES) algorithm. The encrypted bits are then randomized in adata randomization block. A bit-to-octal conversion block converts therandomized binary sequence into a sequence of 3-bit symbols. The symbolsequence is converted into 16 symbol vectors. The term vector generallyrefers to a column vector, which is generally complex. One symbol fromthe LCC is added to form a vector of 17 symbols.

The 17-symbol vector is trellis encoded starting with the mostsignificant symbol (first element of the vector) and is continuedsequentially until the last element of the vector (the LCC symbol). Thisprocess employs convolutional encoding for converting the input symbol(an integer between 0 and 7) to another symbol (between 0 and 15) andmaps the encoded symbol to its corresponding 16 QAM (or 16 PSK) signalconstellation point. The output of the trellis encoder is therefore avector of 17 elements where each element is a signal within a set of 16QAM (or 16 PSK) constellation signals. (The term signal will generallyrefer to a signal constellation point.)

A link maintenance pilot signal (LMP) is added to form an 18-signalvector, with the LMP as the first element of the vector. The resulting(18×1) vector is pre-multiplied by a (18×18) forward smearing matrixyielding an (18×1) vector b. Vector b is element-wise multiplied by an(18×1) gain preemphasis vector yielding another (18×1) vector c. Vectorc is post-multiplied by a (1×32) forward spatial and spectral spreadingvector yielding a (18×32) matrix R(p), where p denotes the trafficchannel index and is an integer. The 32 columns of matrix R results frommultiplying the spectral spreading factor 4 and spatial spreading factor8. The (18×32) matrices corresponding to all traffic channels carried(on the same traffic partition) are then combined (added) for producinga resulting 18×32 matrix S.

Matrix S is partitioned by groups of four columns into eight (18×4)submatrices A₀ to A₇. The indices 0 to 7 of submatrices A₀ to A₇correspond to the antenna elements over which these symbols willeventually be transmitted. Each submatrix is mapped to tones within onetraffic partition. A lower physical layer places the baseband signals indiscrete Fourier transfer (DFT) frequency bins where the data isconverted into the time-domain and sent to its corresponding antennaelements (0 to 7) for transmission. This process is repeated from thestart for the next 48 bits of binary data to be transmitted in the nextforward transmission burst.

FIG. 3 is an architectural block diagram of remote station X operatingas a receiving station. Data tones and LCC tones are received by remotestation antenna X and a receiver 610. Receiver 610 passes the data tonesand the LCC tones to a spectral despreading processor 612 whichdespreads the data tones and LCC tones. The despread signals are thenoutput from processor 612 to a trellis decoder 614. Trellis decoder 614generates data vectors from the despread signals. The data vectors arethen output to a vector disassembly buffer 616. Data for Alice and datato Bob are output from buffer 616 to Alice and Bob, respectively. Datafor Alice and Bob are also input to a CRC generator 618. CRC generator618 computes a new CRC value for every 64 K-bit data block and outputsthe new CRC value with the block number to a buffer within a CRCcomparison processor 620. The receiving station buffers error detectionmessages that are received from the link control channel in CRCcomparison processor 620 so that the error detection messages areaccessible by their block numbers N, N+1, N+2, etc. When the receivingstation receives a data message on the traffic channel, it performs aCRC calculation on the data block in the message with CRC generator 618for obtaining a resulting new CRC value. If the comparison determinesthat there is a difference in the values, then an error signal isgenerated by an error signal generator 622. The error signal can beprocessed and used in several ways by an error processor 630. Forexample, the error signal can initiate a negative acknowledgment signalthat is to be sent from the receiving station back to the sender stationrequesting that the sender repeat transmission of the data block. Theerror signal can also initiate an update in spreading and despreadingweights at the receiving station for improving thesignal-to-interference and noise ratio of the traffic channel. Anotheruse of the error signal is for initiating an alarm used for other realtime control. Yet another use of the error signal is as part of alogging signal for compilation of a long term report relating to trafficchannel quality.

According to the invention, a remote unit includes a standby mode ofoperation and a sleep mode of operation. Normally, the standby mode isthe mode in which a remote unit scans the CLC channel for a CONNECTmessage for the remote unit. The sleep mode of operation provides areduced power consumption operating mode for extending remote unitbattery runtime during an AC power outage condition. During the sleepmode of operation, the remote unit periodically switches between thestandby mode and sleep mode, with the overall effect being a reductionin the average power required by the remote unit.

Delivery of a CONNECT message to a remote unit operating in the sleepmode is scheduled so that the remote unit is in the standby portion ofthe sleep mode. That is, the remote unit is synchronized and ready forreceiving data from the CLC when the base station begins transmitting onthe CLC. In order to achieve synchronization, a system wide Packet Count(PKT_CNT) is used. The basic unit of measure for synchronization is amod [8] PKT_CNT, which is called a subframe count (SUBFRM_CNT). TheSUBFRM_CNT is incremented every 256 PKT_CNTs, or every 6 ms.

The base station and the remote unit both preferably use the leastsignificant 8 bits of the remote unit ID for determining the particularSUBFRM_CNT at which the CLC CONNECT message should be sent to the remoteunit and, simultaneously, the appropriate time at which the remote unitshould be in the standby portion of the sleep mode for receiving theCONNECT message. When the least significant 8 bits of the remote unit IDare used, the remote unit enters the standby mode once every 256subframes and is ready for receiving an incoming call. The particularsubframe that a remote unit will be ready for receiving an incoming callis called the N_(listen) for the remote unit.

To avoid using a remote unit power status database that is maintained ata central location, the sleep mode features of the present invention arepreferably implemented as part of a standard terminating call retrymechanism. That is, when a terminating call request is received at thebase station MAC Layer, the MAC Layer Access Manager proceeds normallythrough a terminating call setup procedure by transmitting a CONNECTmessage on the CLC to the target remote unit. In the situation when thetarget remote unit is operating in the sleep mode at the time of theCONNECT message transmission, the remote unit will generally be unableto process the message. The base station MAC Layer Access Manager willtime-out and retry transmission of the CONNECT message. Preferably, aretry timer T_(r) is nominally set to 72 ms. The base station MAC LayerAccess Manager retries the CONNECT message for a predetermined number oftries that is set by a system manager. Preferably, the retry count is 2.

When the number of retries equals the retry count, the base station MACLayer Access Manager determines that the remote unit is in the sleepmode and, consequently, attempts to deliver the CONNECT message at ascheduled time that is based on the target remote unit ID. The scheduledtime is a subframe occurring N_(listen) subframes after the boundarysubframe for the TDD timing structure.

The base station MAC Layer Access Manager also reserves the CLC slot(s)required for completing the CLC CONNECT message transmission at the timethe N_(listen) subframe number is derived. That is, when the basestation MAC Layer Access Manager has reached its retry count for aCONNECT message and has determined the N_(listen) subframe, CLC slotavailability is examined for reserving the appropriate CLC slot(s) foruse. As an alternative, a remote unit can scan up to 3 CLC slots for aCONNECT message when in the sleep mode so that a base station can selectfrom 3 CLC slots in case a specific slot is unavailable.

FIG. 5 shows a message flow sequence for a terminating call for thesituation when a target remote unit is operating in the sleep mode. TheMAC Layer of the base station receives a terminating call request at 50.At 51, the MAC Layer of the base station sends a CLC CONNECT message tothe target remote unit. Since the remote unit is in the sleep mode, itdoes not receive the CLC CONNECT message and, therefore, does notrespond. Since there is no response from the target remote unit duringthe T_(retry) period 52, the MAC Layer of the base station sends asecond CLC CONNECT message to the target remote unit at 53. The remoteunit does not respond during T_(retry) 54, so the MAC Layer of the basestation determines the N_(listen) subframe for the remote unit using theleast significant 8 bits of the remote unit ID and waits for theparticular N_(listen) subframe at 55. At N_(listen) for the remote unit,the MAC Layer of the base station sends a CLC CONNECT message at 56. AtN_(listen), the remote unit is in the standby mode and ready to receivethe CLC CONNECT message at 57. In response, the remote unit MAC Layersends a CAC_ACK message to the base station at 58.

The following definitions are used for describing the sleep mode ofoperation of the present invention:

T_(sleep)=the time that a remote unit is in a low-power mode (i.e.,sleeping).

T_(sync)=the time required by a remote unit for re-acquiringsynchronization when exiting the sleep mode.

T_(scan) _(—) _(clc)=the time that a remote unit is operating in astandby mode scanning the CLC for a CONNECT message.

T_(standby)=the total time a remote unit is running (i.e.,T_(sync)+T_(scan) _(—) _(clc))

D_(sleep)=T_(sleep)+T_(standby))/T_(standby), that is, the definition ofthe duty cycle of the sleep mode duty cycle.

Since the base station transmits the CONNECT message at the N_(listen)subframe so that the call can be completed, and the remote unittherefore must be ready for receiving the messages on the CLC channel atthe N_(listen) subframe. The remote unit MAC Layer Access Manager iscapable of deriving the N_(start) _(—) _(sync) subframe number andinsures that all hardware required for the remote unit synchronizationand CLC scanning efforts are released from sleep mode at that time. Thisis done, for example, by using a programmable hardware counter 640 thatis clocked in synchronism with the TDD subframe of the system, as shownin FIG. 3. Prior to entering the sleep mode, or at the time the sleepmode is entered, CPU 650 preferably uses the least significant 8 bits ofthe remote unit ID for determining the N_(listen) subframe for theremote unit. CPU 650 loads counter 640 with a value related toN_(listen) and synchronizes counter 640 using a Start Sync signal.Counter 640 provides an interrupt to CPU 650 once every 256 subframes,initiating a re-synchronization process. CPU 650 responds by controllingpower supply 660 to provide power 661 to the various components ofremote unit used for receiving a CLC CONNECT message. CPU 650 alsooutputs an enabling signal to the spectral despreading processor 612 toenable the remote unit to receive messages from the base station.

The remote unit begins its re-synchronization effort at a subframeN_(start) _(—) _(sync) that occurs some determined period of time priorto the occurrence of the N_(listen) subframe. Simulations of the remoteunit synchronization algorithms indicate that a remote unit acquiressynchronization with a base station when exiting a period of sleep in aminimum time of 122 ms and a maximum time of 200 ms. The actual timeadditionally depends on hardware component tolerances, the ambienttemperature and numerous other factors. For the purposes of thisdisclosure, a worst case synchronization acquisition time T_(sync) of200 ms is used. This equates to approximately 34 subframes. Therefore,N_(start) _(—) _(sync)=N_(listen)−34 subframes.

FIG. 6 is a timing diagram showing the sequence of events for a remoteunit operating in the sleep mode. Each vertical line in FIG. 6represents a subframe boundary. The time between each subframe boundaryis 6 ms. A remote unit is shown as being in a sleep mode. At N_(start)_(—) _(sync), counter 640 sends an interrupt request to CPU 650 (FIG.3). CPU 650 responds by controlling power supply 660 to provide power tothe various components of the remote unit needed for receiving a CLCCONNECT message. In FIG. 6, the remote unit is in the sleep mode at 60.At 61, N_(start) _(—) _(sync) occurs and the remote unit resynchronizesfor a number of subframes. Preferably, about 34 subframes are requiredfor a remote unit to reacquire synchronization. At N_(listen), theremote unit scans the CLC channel for any CLC CONNECT messages for theremote unit. The remote unit scans for 2 subframes, as shown in FIG. 6at 62. The remote unit can also be set to scan for a CLC CONNECT messageover a different number of subframes other than 2 subframes dependingupon system requirements. If no CLC CONNECT message is received atN_(listen), the remote unit returns to the sleep mode at 63. If a CLCCONNECT message is received, the call is established in a normal manner.

As a first illustrative example of the timing aspects of the sleep modeof the present invention, the least significant 8 bits of a remote unitID are used so that the N_(listen) cycle time is 1536 ms (256×6 ms). Theremote unit synchronization acquisition time N_(sync) is estimated to be34 subframes (204 ms), and a CLC scan time for 2 CLC subframes ischosen. It follows that,

T _(sleep)=220 subframe times=220×6 ms=1.320 s

T _(sync)=204 ms=34 subframes×6 ms

T _(scan) _(—) _(clc)=12 ms=2 subframes×6 ms

T _(standby)=212 ms

Therefore, the total sleep mode/standby mode cycle time is 1536 ms, andthe total remote unit power-on time is 212 ms. The overall duty cycle is7.25:1. For this example, the maximum delay for delivery of a CONNECTmessage is 1.530 seconds (1536 ms−6 ms). The nominal CONNECT messagedelay delivery time is about 0.766 seconds.

Using a longer delay in CONNECT message delivery time permits the remoteunit to be in the sleep mode for a greater period of time. As anotherexample, the N_(listen) subframe is determined by using the leastsignificant 9-bits of a remote unit ID. Thus, the N_(listen) interval is512 subframes. In this example, even though the sleep time is longer,the maximum synchronization acquisition time T_(sync) remains the same.This is based on the fact that any temperature change of the remote unitis not sufficient for requiring a coarse TDD synchronization to beperformed. It follows that,

T _(sleep)=476 subframe times=476×6 ms=2.856 s

T _(sync)=204 ms=34 subframes×6 ms

T _(scan) _(—) _(clc)=12 ms=2 subframes×6 ms

T _(standby)=212 ms

The total sleep mode/standby mode cycle time is 3072 ms (512×6 ms), andthe total remote unit power-on time is 212 ms. The overall duty cycle is14.5:1. For this example, the maximum delay of delivery of a CONNECTmessage is 3.066 seconds (3072 ms−6 ms). The nominal time for deliveryof a CONNECT message is about 1.536 s.

Table I below summarizes various scenarios:

TABLE I Nominal CLC Battery Sleep CONNECT RU RU Power Runtime Timemessage delay Synchronization Duty Cycle (approx. (ms) time (ms) Time(ms) (approx.) hrs) 1320 663 220 7.25:1   11.5 1320 663 150 9:1 12 28561428 150 18:1  13.5 2856 1428 200 14:5  13 2856 1428 300 10:1  12

The situation of a call originating from a remote unit that is operatingin the sleep mode is straight forward compared to the situation when acall terminates at a sleeping remote unit. That is, the remote unitexits the sleep mode in response to a user command. The originating calldelivery time, i.e., the time taken for delivering an ACCESS message onthe CAC, is delayed by approximately 200 ms since the remote unit mustre-acquire synchronization before the ACCESS message may be transmitted.

In normal system operation, a base station polls remote units at aperiodic rate for determining status of each remote unit. Each remoteunit responds to the Poll Request message with a Poll response messageusing the CAC channel. When a remote unit is in a sleep mode ofoperation, the Poll Request message will not be received and,consequently, the remote unit will not respond with a Poll Responsemessage. The present invention provides two alternatives for handlingsuch a situation from the system point of view. The first approach is toalways schedule a Poll Request message to arrive at a remote unit duringthe N_(listen) subframe for the remote unit whether the remote is in thestandby or the sleep mode. The remote unit will receive the Poll Requestmessage regardless of AC power status. A disadvantage associated withthis approach is that the CAC channel is used by the remote unit for aPoll Response message, causing the remote unit transmitter to be used,effectively wasting battery power when in the sleep mode.

The alternative approach is for a remote unit to ignore the Poll messagefrom the base station during AC power outage situations and allow anOAM&P Layer at the base station to recognize that a non-responsiveremote unit may possibly be in the sleep mode and, consequently, beaware of the power status of the remote unit in questions power.

FIG. 7 is an exemplary graph showing Battery Operating Time, measured inhours, for Sleep Mode Duty Cycle (:1). From FIG. 7, it is apparent thatthe length of time that a remote unit is sleeping has a significantimpact on the run time of the battery. Also, from FIG. 7, it is alsoapparent that the battery run time begins to flatten with duty cycleafter about a 10:1 ratio. Lab results for simulated sleep mode operationwith a new, 7.2 amp-hour battery installed in a prototypeuninterruptable power supply have yielded runtimes between 12 hours, 12minutes to 12 hours, 32 minutes under the conditions that the remoteunit is at room temperature, the sleep mode period is set for 3 seconds,and the sleep mode duty cycle is 10:1 (0.3 s standby state and a 2.7 ssleep state).

A remote unit operating in the sleep mode preferably provides thefollowing characteristics:

Sleep time=2856 ms

RU Synchronization Time=200 ms

Call delivery delay=1428 ms nominally

RU CLC Scan time=36 ms (i.e., three slots for flexibility at Base MACLayer)

Total Cycle Time=3092 ms

Standby Time=236 ms

Duty Cycle=13:1 (approx.)

Battery Operating Time=12.5 hours (approx.)

FIG. 8 is an architectural diagram of the base station as a sender. ThePSTN inputs data to base station Z. The data is sent to the vectorformation buffer 502 and also to the cyclic redundancy code generator504. Data vectors are output from buffer 502 to the trellis encoder 506.The data vectors are in the form of a data message formed byconcatenating a 64 K-bit data block with its serially assigned blocknumber. The LCC vectors output from the CRC generator 504 to the trellisencoder 506 are in the form of an error detection message formed byconcatenating the CRC value with the block number. The trellis encodeddata vectors and LCC vectors are then output to the spectral and spatialspreading processor 508. The resultant data tones and LCC tones are thenoutput from processor 508 to the transmitter 210 for transmission to theremote station.

The base station transmits the CONNECT message at the N_(listen)subframe so that the call can be completed to the remote unit. The basestation knows to send the messages on the CLC channel at the N_(listen)subframe. The base station's MAC Layer Access Manager is capable ofderiving the N_(start) _(—) _(sync) subframe number. This is done, forexample, by using a programmable hardware counter 540 that is clocked insynchronism with the TDD subframe of the system, as shown in FIG. 8.When the base station wants to send a message to the remote unit, theCPU 550 preferably uses the least significant 8 bits of the remote unitID for determining the N_(listen) subframe for the remote unit. CPU 550loads counter 540 with a value related to N_(listen) and synchronizescounter 540 using a Start Sync signal. Counter 540 provides an interruptto CPU 550 once every 256 subframes, initiating a re-synchronizationprocess. CPU 550 responds by outputting an enabling signal to thespectral and spatial spreading processor 508 to enable the base stationto transmit messages to the remote unit when the remote unit is in itsstandby mode.

Still another alternate embodiment applies the above described inventionin the PWAN Frequency Division Duplex Communications System described inthe Alamouti, Michaelson et al. patent application cited above.

Although the preferred embodiments of the invention have been describedin detail above, it will be apparent to those of ordinary skill in theart that obvious modifications may be made to the invention withoutdeparting from its spirit or essence. Consequently, the precedingdescription should be taken as illustrative and not restrictive, and thescope of the invention should be determined in view of the followingclaims.

What is claimed is:
 1. In a wireless communications network, a method ina base station to communicate with a remote unit that is in a sleepmode, the remote unit having a unique identification value, comprisingthe steps of: establishing a periodic reference instant at the basestation and at the remote station; determining a delay intervalfollowing said periodic reference instant at the base station, saiddelay interval being derived from said unique identification value ofsaid remote unit; and transmitting a message from the base station tothe remote unit at a second instant following said delay interval, saidremote unit having changed from said sleep mode to a standby mode aftersaid delay interval; wherein said periodic reference instant isestablished by a beginning subframe count instant that is incremented bya packet count value at the base station and at the remote unit.
 2. Themethod of claim 1, wherein said delay interval is determined by a valueN of a quantity of M least significant bits of said uniqueidentification value of said remote unit, the delay interval being aninterval required for the occurrence of a plurality of N of saidbeginning subframe count instants.
 3. The method of claim 2, whereinsaid remote unit changes from said sleep mode to a standby mode aftersaid delay interval.
 4. In a wireless communications network, a methodin a base station to communicate with a remote unit that is in a sleepmode, the remote unit having a unique identification value, comprisingthe steps of: establishing a periodic reference instant at the basestation and at the remote unit; determining a delay interval followingsaid periodic reference instant at the base station, said delay intervalbeing derived from said unique identification value of said remote unit;attempting to initiate a communication from said base station to saidremote unit; concluding at the base station that the remote unit is insaid sleep mode if said attempting step fails to initiate communicationswith the remote unit; waiting for said delay interval following saidperiodic reference instant at the base station; and transmitting amessage from the base station to the remote unit at a second instantfollowing said delay interval, said remote unit having changed from saidsleep mode to a standby mode after said delay interval; wherein saidperiodic reference instant is established by a beginning subframe countinstant that is incremented by a packet count value at the base stationand at the remote unit.
 5. The method of claim 4, wherein said delayinterval is determined by a value N of a quantity of M least significantbits of said unique identification value of said remote unit, the delayinterval being an interval required for the occurrence of a plurality ofN of said beginning subframe count instants.
 6. The method of claim 5,wherein said remote unit changes from said sleep mode to a standby modeafter said delay interval.
 7. A highly bandwidth-efficientcommunications method in a base station to communicate with a remoteunit that is in a sleep mode, the remote unit having a uniqueidentification value, comprising the steps of: establishing a periodicreference instant at the base station and at the remote unit;determining a delay interval following said periodic reference instantat the base station, said delay interval being derived from said uniqueidentification value of said remote unit; receiving at a base station aspread signal comprising an incoming data traffic signal spread over aplurality of discrete traffic frequencies; adaptively despreading thesignals received at the base station by using despreading weights;attempting to initiate a communication from said base station to saidremote unit; concluding at the base station that the remote unit is insaid sleep mode if said attempting step fails to initiate communicationswith the remote unit; waiting for said delay interval following saidperiodic reference instant at the base station; and transmitting at thebase station to the remote unit a spread signal comprising an outgoingdata traffic signal spread over a plurality of discrete trafficfrequencies; wherein said periodic reference instant is established by abeginning subframe count instant that is incremented by a packet countvalue at the base station and at the remote unit.
 8. The method of claim7, wherein said delay interval is determined by a value N of a quantityof M least significant bits of said unique identification value of saidremote unit, the delay interval being an interval required for theoccurrence of a plurality of N of said beginning subframe countinstants.
 9. The method of claim 8, wherein said remote unit changesfrom said sleep mode to